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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Common Stock |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 873.4 | $ 739.7
Trading Securities, Unrealized Holding Gain | 171.1 | 129.4
Trading Securities, Unrealized Holding Loss | (1.0) | (3.0)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 1,043.5 | 866.1
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 303.8 | 246.6
Trading Securities, Unrealized Holding Gain | 34.8 | 6.8
Trading Securities, Unrealized Holding Loss | (47.2) | (39.7)
Net foreign currency gains (losses) | (1.7) | (4.9)
Fixed maturity investments, at fair value | $ 289.7 | $ 208.8

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